Many commentators expect Apple to have ditched the headphone jack, leaving only one port.

The previous model - the iPhone 6S - was the world's bestselling handset, according to new research.

The study, by Strategy Analytics, said 14.2 million copies of the phone shipped between April to June, accounting for 4% of the market. By contrast, the Samsung Galaxy S7 Edge shipped 8.3 million units over the same period.

However, both fell short of the earlier iPhone 6, which shipped 26.3 million units over the same months in 2015.

"Smartphone growth is sluggish at the moment due to ongoing economic volatility worldwide, high ownership penetration in most major countries, and a lack of new innovation from device manufacturers," commented the market research firm's director Linda Sui.

Two lenses

Dropping the headphone jack in the iPhone 7 would encourage consumers to use Bluetooth headphones or buy those compatible with Apple's Lightning port, which is also used for charging.

However, old gear would not be completely obsolete, as Apple is expected to include an adapter jack.

Image copyrightGetty ImagesImage caption
The new phone is likely to stick with two sizes of the iPhone 6

Camera quality is expected to be improved and this time round the upgrade might mean a two-lens clicker for some of the bigger models.

Other handsets' dual-lens camera can take pictures with two different exposures, then combine the two images for improved picture quality - but it is unknown how Apple would implement the feature.

Analysis: Dave Lee, North America Technology correspondent

Another year, another iPhone. But then, it's hardly just been "another year" for Apple, has it?

Distractions, lots of them. And it doesn't look like the new iPhone is going to deflect much of that attention away.

While we're used to saying "evolution, not revolution" in the smartphone market, this upgrade is not expected to turn many heads nor send customers running to the phone shop to upgrade.

If, as we expect, the headphone jack is removed from the device, expect a lot of anger from those who don't want to be ushered into buying wireless headphones.

Then again, Apple faced the same kind of anger when it rolled out the smaller "lightning" charging port, but everyone's just about got over that. Take a look at the "old" Apple charger and tell me it doesn't look almost comically massive. In hindsight, the right call.

A better camera would go down extremely well - more photographs are taken with iPhones than any other device. But some reported leaks suggest only the larger, less popular iPhone size will get the new camera technology.

Looking a bit further ahead, some are speculating that Apple is holding back this time around, instead saving new features for 2017 and what will be the iPhone's 10th anniversary.

Other expected change include increased storage and better speakers.

The new phone is also said to be water resistant, handling submersion for up to 30 minutes - a feature already offered by several of Apple's Android competitors from the likes of Samsung or Sony.

Apple is thought to be sticking with its two sizes, meaning there would be an iPhone 7 and an iPhone 7 Plus.

Neither is the design thought to be changing much, giving it the same overall look as the current models.

Rumour has it there will be more colours available, for instance reintroducing black as an option.

While the new models will be revealed to the world at the launch, consumers will have to wait a few weeks before they are shipped. Pre-orders are expected to open on Friday.

The timing still is crucial. Samsung beat Apple by releasing its Galaxy Note 7 last month. The device was well received by users and critics alike and started selling well.